EROSIONAL IMPACTS OF TIMBER HARVEST AND EXTREME HYDROLOGICAL EVENTS IN A HIGH-RELIEF MAINE WATERSHED

Flooding in mountainous watersheds of the northeastern United States is an important mechanism for supplying sediment to river channels and maintaining ecosystem services. In contrast, increased erosion can pose threats to natural resources, water quality, and built infrastructure. Consequently, an improved understanding of the origin of and response to both natural and human landscape disturbances is needed to better manage their impacts. We focus on the Kennebago River watershed upstream of Little Kennebago Lake, northwestern Maine. Logging operations in this area began in the late 19th century and continue to the present. Using sediment cores collected from Little Kennebago Lake we examine evidence for the deposition of terrestrial sediment mobilized by floods. Cores collected before and after an extreme rainfall event on July 3, 2018 provide direct observations of the depositional signature of a known flood event. Primary analyses focus on bulk composition determined through loss on ignition, magnetic susceptibility, scanning X-ray fluorescence, and pollen analysis. Radiocarbon dates and short-lived radioisotope profiles indicate that the cores record sedimentation from 800 CE to the present and highlight changes in the rate of sediment delivery to the lake. We also study watershed sediment sources using digital elevation models, satellite images, and historical aerial photographs. The erosional response to the July 3, 2018 rainfall event, in comparison with other historical events from rainfall and streamflow records, and the general scarcity of flood deposits in the cores prior to the twentieth century help constrain the sensitivity of the watershed to extreme hydrologic events. We preliminarily interpret a 20th-century increase in the frequency of event deposits and overall increase in the delivery of clastic sediment to Little Kennebago Lake as a response to historical logging, and likely the process of driving sawn logs downstream through artificially enhanced floods as done in the early part of the century. Despite a decrease in event deposits over the past few decades, continued accumulation of terrestrial sediment at elevated rates suggests either a lasting legacy of historical practices or ongoing impacts related to current logging, possibly compounded by recent climatic conditions.